Khamey

My group is trying to navigate a lawn mower in an open field using one of two methods.

Having two GPS units and using what we are referring to as relative GPS:Essentially we have one stationary GPS that assumes the first point it reads is the point it is at and all other data it receives has error in it. It calculates this error and sends it over to the "rover" so that it may calculate what the error is and subtract it from its own coordinates. We are assuming since the units will be in a 30 m radius the errors received by both GPS will be similar. Is this a correct assumption. Is there a way to get <1m accuracy using 2 GPS units?

The second method is using an accelerometer, gyroscope, and single GPS combined with a Kalman filter.While there is information on combining the accelerometer and gyroscope, I'm not sure how to begin filtering the the GPS into this system.

Any advice is appreciated on how to waypoint navigate our lawn mower with a minimum of 1m of accuracy.

Essentially we have one stationary GPS that assumes the first point it reads is the point it is at and all other data it receives has error in it. It calculates this error and sends it over to the "rover" so that it may calculate what the error is and subtract it from its own coordinates.

After you try this please come back and tell us how well it worked. I'd considered trying it out myself but ... too many projects.

Khamey

So far it seems that the errors that each GPS gets are independent of each other. At about a foot apart our Due calculates the approximate distance between the two is anywhere between 9m and 90m which makes no sense. Worse case scenario is that they should read about 20m apart assuming each GPS has 10m accuracy and each is assumed to be right next to each other at first reading then eventually drifts to the farthest part away of its accuracy.

Khamey

After being outside a while the readings went from about 9m to 0m when they were about 1m apart. I think it's safe to assume the two GPS receive different errors therefore making the an infeasible solution. You keep mentioning differential GPS but I don't know how that would work, is it significantly different than what I am trying to do?

Don't you need a surveyed point for the GPS to compare with to do differential?

You should only need a surveyed position if you want absolute coordinates. If you just want relative coordinates ("Where am I relative to the lawn boundary?") I think you just need a fixed location that both agree on. I would think that once both units have current ephemeris data they would agree on where they are.

If the base station is going to be in the same place each time you could calculate a long-term median position to use as the 'known' position.

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Khamey

Do we put the GPS units in the same location until they agree on a coordinate then start doing calculations? Or are we surveying for a set time until they have a good average location and then doing locations. Things are clearing up and we are doing a test today. Your advice is very helpful.

I don't think you can do "differential GPS" just with two regular GPS modules. You need a special devicewhich is in the thousand dollar ballpark rather than the $50 ballpark.

I have done a bunch of experiments with two regular GPS modules to see if I got a more precise locationfrom them, and the answer seems to me, to be no. You do get some situations where one has a fixand the other doesn't have a fix, but in terms of precision, not much improvement over the normalposition distribution noise.